The present invention relates to a method for measuring characteristic parameters of living tissues by transmitting ultrasonic waves into a living body and analyzing reflected waves therefrom. More particularly, the invention pertains to a method for measuring the frequency dependency of the reflection coefficient and that of the attenuation coefficient of the living tissue separately of each other.
Conventional systems for obtaining tissue characteristics by analyzing reflected ultrasonic waves of plural frequencies have been proposed by Iinuma (Japanese Patent "Kokai" No. 38490/74) and Nakagawa (Japanese Patent Publication No. 24798/77). With these systems, however, their operations are based on sound pressure waveforms, to that when the ultrasonic waves have a wide frequency band, like pulses, accurate measurements are impossible under the influence of the phase relationships of respective frequency components, pulse overlapping of continuous reflected waves and phase cancellation in a receiving sensor.
The abovesaid prior art systems can be employed in the case where the living body is composed of several kinds of tissues, an ultrasonic reflector of a definite, approximately smooth surface exists at the boundary between adjacent tissues and the reflection factor and the transmission factor of the ultrasonic reflector have no frequency dependence. Such reflection is called specular reflection.
With recent technological progress, however, it has become possible to measure a weak reflection from the tissue between boundaries. In general, the tissue has such a microstructure that cells, capillary vessels, lymphatic vessels, muscular fibers and so forth intertwine complicatedly. A typical size of such a tissue is nearly equal to or smaller than the wavelength of ultrasonic waves. On account of this, reflected waves from the microstructure are accompanied by complex interference owing to phase dispersion and pulse overlapping, introducing in a B-mode tomogram a speckled pattern commonly referred to as "speckle". It has been proven experimentally that reflection from the tissue (backward scattering) has a frequency characteristic such that its power reflection coefficient is proportional to the nth power of the frequency, and that the value of n is a characteristic value (a parameter) representing the tissue. It has been reported that n=2.1 to 2.2 in the liver and n=3.3 in the myocardium.
Systems for obtaining the tissue characteristics in such a case have been proposed by Hayakawa in references 1* and 2* and by others. FNT *1. "Theory of Reflecting Ultrasonic Computer Tomograph Using Plural Frequencies", Proceedings of the 37th meeting of Japan Society of Ultrasonics in Medicine, in Japanese FNT *2. "Multifrequency echoscopy for quantitative acoustical characterization of living tissues.", J. Acoust. Soc. Am. 96 (6), June 1981.
Noting the energy value of ultrasonic waves, the system *1 conducts a second order differentiation of an attenuation coefficient by the natural logarithm of the frequency (.differential..sup.2 .alpha./.differential..sub.(lnf).sup.2) and a first order differentiation in the direction of depth, by which "a second order differentiated value of the attenuation coefficient of the ultrasonic waves by the natural logarithms of their frequencies" is obtained as a tissue characteristic parameter. According to the system *2, energy (or power) values of the ultrasonic waves are obtained through utilization of three frequencies f.sub.1, f.sub.2 and f.sub.3 and, as a difference value, "the second order differentiated value of the attenuation coefficient of the ultrasonic waves by the natural logarithm of their frequencies" is obtained in the form of a parameter. As experimentally ascertained, it is indicated that, when the attenuation coefficient is proportional to the first power of the frequency, as experimentally ascertained, the abovesaid parameter ##EQU1## becomes proportional to the attenuation constant .alpha..
The abovesaid Hayakawa system requires complex processing corresponding to the second order differentiation by the natural logarithm of the frequencies, and hence is difficult realtime processing and poor in SN ratio; further, tissue information on the reflection (backward scattering) is entirely lost. Moreover, the parameters thus obtained are insignificant from a physical viewpoint.